Para-xylene is useful for the production of terephthalic acid. Terephthalic acid is used for the production of polyester resins and fibers such as Dacron. Para-xylene is a byproduct of reforming processes for the manufacture of fuels, particularly for the manufacture of gasoline. Para-xylene occurs in approximately equilibrium amounts, about 22%, in the C.sub.8 aromatics fraction of a typical reformate stream. (All %s mentioned herein are weight %s.)
Para-xylene and other aromatics can be recovered from reformates using a combination of extraction and distillation steps. Para-xylene can be separated from other C.sub.8 aromatics and purified by processes which include crystallization or adsorption processes. The other C.sub.8 aromatics, which now contain para-xylene in less than equilibrium amounts, are typically recycled through an isomerization unit in order to bring the para-xylene percentage back up to near equilibrium levels. The resulting stream is then recycled to the separation and purification unit to recover additional para-xylene.
The efficiency of para-xylene recovery and purification would be increased and the cost would be decreased if the C.sub.8 aromatics feedstream was enriched in para-xylene. In the best case, with a very rich para-xylene stream, the recycle through the isomerization step could be eliminated entirely. Since conventional reforming and aromatization produces para-xylene at only approximately equilibrium levels a conventional reforming or aromatization process does not give the desired improved efficiency of para-xylene recovery. A reforming/aromatization catalyst and process which would give C.sub.8 aromatics which are enriched in para-xylene above equilibrium levels would thus be highly desirable.
It is known to produce xylenes with paraxylene levels above equilibrium by disproportionation of toluene. This process produces xylenes and benzene by a transalkylation mechanism. Patents which illustrate the production of para-xylene rich xylene streams from the disproportionation of toluene include U.S. Pat. Nos. 4,011,276; 4,016,219; 4,052,476; 4,029,716; 4,067,919; 4,097,543; 4,098,837; 4,127,616; 4,160,788; 4,182,923; 4,361,713; 4,365,104; 4,367,359; 4,370,508; 4,308,685 and 4,384,155, all of which are incorporated by reference.
U.S. Pat. No. 4,007,231 discloses the use of an antimony oxide modified zeolite, for example, HZSM-5, to produce para-xylene rich C.sub.8 aromatics from toluene and from C.sub.3 -C.sub.10 paraffins, olefins, or naphthenes. Also, U.S. Pat. No. 4,117,026 utilizes an intermediate pore zeolite, for example, HZSM-5, which has been modified by metal oxides, precoking or using a large crystal form to produce para-xylene rich C.sub.8 aromatics from toluene or high molecular weight olefinic or paraffinic feeds. In this patent a dodecane feed shows 63% para-xylene enrichment of the C.sub.8 aromatics. The catalyst does not contain a platinum group component and is not utilized for processing a C.sub.5 -C.sub.9 predominantly paraffinic/olefinic hydrocarbon feedstock.
Some catalysts are known to be shape selective for the isomerization of ethylbenzene into para-xylene rich xylenes. Isomerization catalysts and process of this nature are set forth in U.S. Pat. No. 4,101,595; 5,028,573; 5,082,984; 4,783,571 and 5,043,512.
None of the prior art is concerned with an aromatization process or a reforming process which will produce acceptably high yields of para-xylene rich xylenes starting with a C.sub.8 non-aromatics containing feed such as a typical reformer feedstock.